1. Field of the Invention
The present invention relates to a reinforced polypropylene composition possessing excellent vibration damping qualities over a wide temperature range, excellent mechanical strength, and excellent heat resistance.
2. Description of the Related Art
Numerous attempts have recently been made to substitute plastic products for various metal automotive components in order to reduce weight and realize an improvement in fuel economy. Among plastic materials, polypropylene is a material which is well balanced in various physical properties and relatively inexpensive, so that it is often applied to many trim parts for automobiles. A reinforced polypropylene obtained by blending polypropylene with a filler such as talc, glass fiber, calcium carbonate, or a blend thereof, is both strong and heat resistant. It is thus often utilized in automotive engine compartment components such as timing belt covers, gear covers, fan shrouds, or air cleaner covers, so as to impart heat resistance in a high temperature atmosphere.
In addition to vehicle weight reduction, however, automotive noise reduction has been sought. In particular, a reduction in noise originating in the engine compartment has been desirable.
As a measure to counter noise, vibration damping, sound insulation, vibration proofing, and sound absorption have been proposed. The measure should be selected in view of the fact that either sound propagated in a solid or sound transmitted in air may contribute to the engine noise. In automotive parts such as a timing belt cover or a gear cover, the noise is caused primarily by sound propagated in a solid. Thus, an improvement of the vibration damping property in plastic automotive materials is desirable for the purpose of noise reduction.
Japanese Patent Application Laid Open No. 43443/1987 discloses a conventional reinforced polypropylene with improved vibration damping properties. This reinforced polypropylene is produced by blending polypropylene with a tackiness-providing product such as a cumarone-indene resin or thermoplastic elastomer.
In the above-mentioned reinforced polypropylene, however, the mechanical strength, heat resistance, and vibration damping property are not sufficient in the case where the conventional reinforced polypropylene is utilized in automotive parts employed in a relatively high temperature environment ranging from 40.degree. to 100.degree. C.
In general, the molecular structure of plastic (resin) materials is irregular and the linkage between chains is weak in comparison to metallic materials. Plastics are thus highly viscoelastic and exhibit vibration damping properties. The vibration damping property of such materials is highly temperature dependent, however, so that damping is greatest near temperature at which the loss factor of dynamic dispersion due to the glass transition of the resin material is a maximum. In general, the damping property is greatly lowered at a temperature in the range of practical uses. The maximum value of the loss factor due to the glass transition appears in general at a temperature ranging from -30.degree. to 25.degree. C., and the minimum value appears at about 50.degree. C.
This property appears similarly in a reinforced polypropylene containing a filler. The value of the loss factor is even greater because of the addition of the filler. Thus, the lowering of the value of the loss factor is a reason why a satisfactory vibration damping property cannot be expected in a reinforced polypropylene used primarily at a temperature ranging from 40.degree. to 100.degree. C.